Ship-borne gravity stabilized antenna

ABSTRACT

A gimballed-platform-mounted antenna arrangement for semistabilizing the orientation of the antenna of said arrangement on a ship, said arrangement being semi-stabilized in use by means of its having a long natural period of oscillation and a high moment of inertia as hereinbefore defined.

United States Patent 1191 Pope et al. 1 Jan. 14, 1975 [5 SHIP-BORNEGRAVITY STABILIZED 1,569,325 1/1926 Leib 343/764 ANTENNA 2,477,5748/1949 Braddon 343/765 2,599,381 6/1952 Gerks 343/765 [75] Inventors:Dona G y p Lilley 2,901,208 8/1959 Jones 343/765 Bottom; Rodney JohnKirkby, 3,789,414 1/1974 Bauer et a1. 343/872 Chesham, both of England[73] Assignee: The Post Office, London, England Primary Examiner-EliLieberman Attorney, Agent, or Firm-Kemon, Palmer & [22] Filed. Nov. 26,1973 Estabrook [21] Appl. No.: 418,908

[57] ABSTRACT [52] US. Cl 343/709, 343/765, 343/872,

243 1 2 A g1mba1led-platf0rm-mounted antenna arrangement 51 1111.01.H0lq 1/34- for Semi-Stabilizing the Orientation of the antenna of [58]Field of Search 248/182; 343/709, 765, Said arrangement on a Ship, Saidarrangement being 343/7 372 semi-stabilized in use by means of itshaving a long natural period of oscillation and a high moment of in- 5References Cited ertia as hereinbefore defined.

UNITED STATES PATENTS 5 Claims, 6 Drawing Figures 1,260,181 3/1918Garnero 248/182 SHIP-HORNE GRAVITY STABILIZED ANTENNA This inventionrelates to an antenna arrangement for a ship and in particular to meansfor maintaining the orientation of the antenna within predeterminedlimits.

A ship-borne antenna system for a maritime satellite communicationsservice should have a high gain in order to minimise the satellite powerrequirements (and hence satellite costs) for the satellite-to-shipcommunications link. However, as the gain increases, the beamwidthnarrows and the allowable limits of the antenna orientation converge.Any ship at sea will, in anything other than a flat calm, roll and pitchto some extent and clearly if the degrees of roll and pitch are greaterthan the beamwidth of the antenna system the antenna orientation willneed to be controlled if an adequate communications capability is to bemaintained.

For a single helix antenna, the beam-edge gain relative to isotropic is6 dB, and the corresponding figures for a 1 m diameter dish and a 2% mdiameter dish are 18 dB and 26 dB respectively. The beamwidths (to the 3dB points) of these antennae are 64, and 6 respectively. Trawlers areknown to roll to more than i 35 and hence some control on the antennaorientation would be required even when using a modest 6 dB gain singlehelix antenna.

It is known to stabilize an antenna platform by means of gyroscopes andservo-systems but these are expensive and achieve stabilities of theorder of 2*: which is much more stable than is required even for a 2% mdiameter dish antenna, which would be impractically large for manyships. It is also known to provide a low gain antenna without any formof stabilisation, but as mentioned above this leads to higher satellitecosts. The present invention seeks a compromise solution whereby amoderately high gain antenna may be controlled, in almost all seaconditions, to within about 5 of its desired orientation. An antennathus controlled is referred to hereinafter as being semi-stabilized; anantenna controlled, e.g. gyroscopically, to within about i V2 is hereinsaid to be staiblized.

Broadly considered, the invention provides a gimballed-platform-mountedantenna arrangement for semi-stabilizing the orientation of the antennaof said arrangement on a ship, said arrangement being semistabilized inuse by means of its having a long natural period of oscillation and ahigh moment of inertia as hereinbefore defined.

A long period of oscillation" is hereby defined as a period ofoscillation longer than the longest significant period of the periodiccomponents of motion the ship in the sea. A high moment of inertia ishereby defined as a moment of inertia sufficiently large that theamplitude of angular periodic motion induced in the antenna arrangementby translational periodic motion of the ship in the sea is at most ofcomparable magnitude to the amplitude of angular periodic motion inducedin the antenna arrangement by angular periodic motion of the ship in thesea.

The natural period of oscillation may be at least two, and preferablytwo and a half, times the longest significant period of oscillation ofthe ship in the sea so that the degree of coupling from the motion ofthe ship to that of the arrangement will be reduced.

In the preferred form, the invention comprises a structure of mass M kgincluding a directional receiving antenna, said structure beingpendulously mounted upon the ship by hearing means having two axesintersecting at a pivotal centre, the arrangement being such that abouteach axis the total Coulomb friction torque T Nm of the bearing means isrelated to the moment of inertia I kgm of the structure by theexpression T/! s 7.5 X 10" N/kgm and the center of gravity of thestructure is displaced below the pivotal center by a distance L, where L2 X 10 I/M m.

The structure may include means for directing the antenna to apredetermined orientation, the directing means preferably includingazimuth directing means for directing the antenna in azimuth andelevation directing means for directing the antenna in elevation. Theazimuth directing means can, in use, be operatively associated with acompass of the ship so that the azimuthal setting of the antenna ismaintained, thus to compensate for yaw and changes of course of theship.

The invention will now be described by way of example only withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the phase relationshipbetween transverse, translational, and angular periodic motions of aship and angular periodic motion induced thereby in a pendulousstructure mounted on the ship;

FIG. 2 is a diagrammatic representation of the relationship betweenantenna beamwidth and required stability for a pendulously mountedantenna;

FIG. 3 is a schematic representation in front elevation of an antennaarrangement according to the invention;

FIG. 4 is a partial side elevation corresponding to FIG. 3;

FIG. 5 shows a tested gimballed platform assembly for an antennaarrangement according to the invention; and

FIG. 6 is a partial cross-section through the assembly of FIG. 5. v

At sea, in anything other than a flat calm, a ship undergoes afluctuation which can be resolved into periodic components of motion.The significant fluctuations, at least as regards the present invention,are

known as pitch, surge, roll and sway. Pitch is an angular fluctuationabout a beam-wise axis. Surge is a longitudinal translationalfluctuation, that is to say movement along a line ahead/astern. Roll isan angular fluctuation about a longitudinal axis and sway. is abeam-wise translational fluctuation.

Referring to FIG. 1, the ship as indicated at 10 is at one extreme ofits sway movement, i.e., it is at its maximum displacement from itsgeneral line of progress in the plane indicated at 11. At this point,its acceleration is in the direction of arrow 12. Sway, which ariseswhen the ship is in a beam-sea with the waves meeting it sideon, iscaused partly by the tendency of the ship to slide down the wave andpartly by the circular motion of the water in the wave, and occurs inassociation with rolling of the ship so that by the time the ship passesthrough the plane 11, it has taken up an attitude as indicated at 13. Bythe time the ship has reached its other extremity of sway as indicatedat 14, it is again substantially vertical. At this other extremity it issubject to an acceleration in the direction of arrow 15.

When the ship is in the position indicated at 10, its roll is in thedirection of arrow 16. In position 14, its roll is in the direction-ofarrow 17.

Any pendulously mounted structure in or on the ship will be subjected totorques by the fluctuating motions of the ship, and from FIG. 1 it isclear that both roll and sway will simultaneously give torques in thesame direction, i.e., with the ship in position 10, the torques due toboth sway and roll will be in the direction of arrow 18, and in position14 the torques will both be in the direction of arrow 19. The roll andsway of the ship will thus combine to excite the pendulous structureinto fluctuant motion.

Further, unless the pendulous structure is pivotally mounted about theroll-centre of the ship, that is to say the line about which the shiprolls, the roll of the ship will cause the pendulous structure to havean additional beam-wise translational fluctuation.

The amplitude of fluctuations induced in the pendulous structure by thesway of the ship can be reduced by providing a frictional damping torqueat its pivot. This would, however, increase the coupling between angularmovement of the pendulous structure and angular movement of the ship sothat the amplitude of fluctuation induced in the pendulous structure bythe roll of the ship would be increased. It is therefore necessary toseek some means, other than controlling the pivot friction, which willlimit the sway-induced fluctuation without simultaneously increasingthe. rollinduced fluctuation. In the present invention, this isaccomplished by arranging the pendulous structure of thegimballed-platform-mounted antenna arrangement to have a high moment ofinertia and a long period of oscillation as hereinbefore defined.

The relationship between pitch and surge is similar to that between rolland sway described above and a consideration of the relationship leadsto a similar conclusion, namely that the pendulous structure must have ahigh moment of inertia and a long period of oscillation if theamplitudes of fluctuations induced therein by the motion of the ship areto be limited.

The required limitation on the amplitude of fluctuation of agimballed-platform-mounted antenna arrangement can be determined byconsidering FIG. 2. In FIG. 2, the line 20 represents the actualorientation of a communications satellite relative to the ship and theline 21 represents the orientation of the antenna, at one extreme limitof its angular oscillation. The angle A is the allowable angularstability of the antenna, the angle B is the beamwidth of the antenna tothe 3 dB points and angle C is an angular tolerance obviating the needfor continuous resetting of the antenna pointing direction. A fast shipsteaming on a great circle route can travel every hour through adistance large enough for the orientation of a geo-stationary satelliterelative to the ship to change by as much as 1 per hour. It is assumedthat it is reasonable to require that the direction of the antenna bere-set every hours and thus an angular tolerance of 5 must be includedin estimating the required stability of the antenna. In other words, itis assumed that the angle C equals 5.

From the geometry of FIG. 2: A (B5 )/2.

As stated previously, the beamwidth of a l m diameter dish antenna isand it follows from this that the required stability for such an antennais i 5.

It can be shown by means of a computer simulation that this degree ofstability can be achieved with an antenna orientation arrangement in theform of a pendulous structure having the following parameters:

Total Moment of Inertia I 2,000 kg m Mass of pendulous structure XDistance of center of gravity of the structure from the pivot, ML 4 kgm,

Total Coulomb friction torque between the ship and the antenna platformsystem, T= 1.5 Nm.

An antenna arrangement with parameters of these values will achieve thedesired stability of i 5 when borne on a cargo ship of 14,000 tons deadweight, beam-on to the prevailing sea in a Beaufort 9 wind.

A system with parameters of these values will also attain the desiredstability when mounted on a trawler in similar sea and wind conditions.It is expected that the desired stability could be maintained under theabove described adverse conditions, for more than 99 percent of thetime.

Because the parameters ML, 1 and T occur in the equations of motion onlyas ratios, these parameters can be converted into a further pair ofparameters TH and ML for which the values should be equal to the ratiosof the base parameters given above.

That is, T/] 1.5/2000 7.5 X 10 N/kg m and ML/] 4/2000 2 X 10' m Thevalues of these parameters are the limiting values for a system whichwill attain the desired stability.

The parameter ML/I is easily arranged to have the correct value sincethe parameter L, being simply the distance between the center ofoscillation and the center of gravity of the system, may be varied atwill. The parameter T is therefore the critical factor. A needle rollerbearing of nominal diameter about 4 cm will give a value of TH equal tothe limiting value above and since the loads imposed on the bearing by asystem with parameters of the above values can be adequately supportedby a needle roller bearing having a diameter of only about 0.5centimeters, it follows that one can arrive at a suitable design byusing needle roller bearings of diameters between these limits.

Referring now to FIGS. 3 and 4, these show schematically an antennasemi-stabilized orientation arrangement having parameters which satisfythe foregoing requirements. The antenna structure, indicated generallyat 23, is in the form ofa l m diameter dish 24 pivotally mounted at 25at the upper end of an up-standing arm 26. Elevation directing means inthe form of a remotely controlled electric motor indicated at 35a isprovided for rotating the dish 24 about the pivot 25 to enable itselevation to be set and re-set. A support 27 is rigidly secured to thelower end of the arm 26.

The antenna assembly comprising the dish 24, arm 26 support 27 and motor35a is mounted upon a platform 28 and azimuth directing means in theform of a remotely controlled electric motor 35b is provided forrotating the antenna assembly about an axis normal to the interfacebetween the support 27 and the platform 28.

As will be apparent from the following description, the interfacebetween the support 27 and the platform 28 will be substantiallyhorizontal so that the azimuth directing means will be operable torotate the antenna assembly about a generally vertical axis. The azimuthdirecting means is coupled to the ships compass and thereby controls thebearing of the antenna to compensate for yaw and changes of course.

The platform 28 is gimbally mounted by means of a universal jointassembly 29 upon some rigid part of the structure of the ship indicatedat 30. The part 30 may be a mast or may be part of the bridge structure.The universal joint assembly 29 has a pair of orthogonal axes arrange tolie respectively parallel to the major horizontal axes of the ship,i.e., the longitudinal (ahead/astern) axis and the beam-wise axis.However it will be appreciated that a gimbal mounting such as auniversal joint may be regarded as having axes intersecting at a pivotalcenter," by which is meant a point about which a structure mounted onthe joint is constrained to rotate.

Extending outwards from the platform 28 are four limbs 31 arranged intwo orthogonal planes.

A weight 32 is secured to the outer end of each limb 31. The fourweights 32 are of substantially equal mass and serve a two-fold purpose:they are of sufficient mass and suitably disposed to give thearrangement a pendulous structure pivotally mounted on the universaljoint assembly 29 and having a long period of oscillation; and theirmasses and their distances from the pivotal centre of the universaljoint assembly 29 are such that the arrangement has a high moment ofinertia. The dish 24 is substantially counter-balanced by acounterbalance mass 33 so that changes in the elevation of the dish willnot significantly alter the moment of inertia of the arrangement or thecentre of gravity of the struc ture.

Each of the weights 32 is of mass 25 kg and they are arranged so thatthe centre of gravity of the arrangement lies approximatelythree-fourths mm below its pivotal center. Such an arrangement has amoment of inertia of approximately 30 kg m and a period of oscillationof approximately 45 s.

The arrangement is covered by a radome indicated at 34 which preventswind forces disturbing the setting of the antenna and protects thearrangement from the maritime environment. The radome may be constructedof any material which is structurally adequate and transparent toelectromagnetic radiation of the working frequency, which in the presentcase is about 1.6 GHz.

The described arrangement is such that the antenna may be stabilized towithin i 5 for ships ranging from trawlers to tankers in virtually inall sea conditions. This degree of stability would enable the ship touse an antenna as large as a l m diameter dish, which is considered tobe a sensible upper size limit for antenna to be fitted in a suitableposition to the majority of oceangoing vessels. Even if the fulltheoretical stability cannot be realized in practice, stabilization towithin about :t 7? should be achievable, and this would allow the use ofa quad helix antenna having a beam-edge gain of 16 dB. Use of a quadhelix antenna would have the advantage that the size of the radome, andhence the cost of the assembly, can be reduced. If the ship is subjectto periodic motions other than the significant motions referred tohereinbefore, it is considered that the period of such motions willeither be so short (e.g., vibration from the ships engines) as to havean insignificant effect upon the stability of the arrangement or so long(e.g., tidal movements of the sea) as to feed energy into the system ata rate which can be dissipated by the pivot friction. It will also benoted that the moment of inertia of the structure is sufficiently largefor the effect of, for instance, variations in the bearing friction andthe resilience of the motor leads 36 to be negligible.

FIG. 5 shows a gimballed platform assembly which has been tested. Theplatform comprises a bearing housing 37 and four weighted arms 38extending therefrom. The upper face of the housing 37 is adapted toreceive and carry a support such as that indicated at 27 in FIGS. 3 and4. A weight 39 is screwed on to the end of each arm 38 and is locked ina desired position thereon by means of a lock nut 40. Each weight 39 islead-filled and has a mass of approximately 30 kg. The arms 38 arearranged in two coaxial, mutually orthogonal pairs and the weights 39 ofeach pair are about 1.4 m apart.

The platform is supported by means of a universal joint within thehousing 37 upon a stanchion 11 which is adapted to be rigidly secured tosome part of the superstructure of a ship. One link of the joint issecured to the upper end of the stanchion 11 and the other is securedagainst the underside of a spacer 42 shown in FIG. 6. It will beobserved that spacers of different vertical dimension can be employed tovary the position of the platform and antenna assembly verticallyrelative to the pivotal center of the universal joint, so that thearrangement is adjustable to cater for antennae of differing size andform or, where desired, to balance the platform alone. As tested, aspacer was used which located the center of gravity of the assembly 075mm below the pivotal center of the universal joint. The joint employedwas a proprietary item having needle roller bearings of about ll mmnominal diameter. The moment of inertia of the tested assembly was alittle over 29 kg m.

In the tests the stanchion 11 was secured on the flying bridge of anunstabilized ship of approximately 1,450 tons deadweight fitted with agyro-horizon device giving an accurate reference to a true horizontalplane. The platform was instrumented using low-friction angular positiontransducers and the outputs therefrom were arranged to be combined withthe outputs of the gyro-horizon device to yield information on theinstantaneous rolland pitch of the platform relative to the truehorizontal plane. The instantaneous angles of roll and pitch of theplatform were subsequently combined vectorially to give the absoluteangle or tilt of the platform at each instant. The tilt angles weresubjected to statistical analysis and a histogram was plotted.

The histogram shows that the platform tilt angle exceeded i 5 for only0.15 percent of the time during which results were recorded, although itshould be noted that this is, of course, strongly dependent upon thetemporal variation of the sea conditions during that time. A truerindication of the stabilizing effect is given by the fact that, inpitch, the ship exceeded i 5 for 0.13 percent of the time while thepitch (that is, fore and aft) angle of the platform exceeded 5 for only0.05 percent of the time. Further, whilst the pitch angle of the shipexceeded i 10 for 0.015 percent of the time, the pitch angle of theplatform exceeded 10 for only 0.001 percent of the time. It is apparentthat the platform was markedly more stable than the ship.

A subsequent examination of the tested assembly showed that the frictionin the bearings of the universal joint was greater than predicted.Better quality bearings would, it is believed, have improved theperformance of the assembly. A further improvement is anticipated fromthe employment of a universal joint of the 5 constant velocity type. Thejoint used in the tests was of the links were mutually inclined. Thusthere would be a variation in angular velocity of the upper link (thatis, the link secured to the platform) about its axis and consequently acouple about that axis, and the gyroscopic effect would then manifestitself as movement around another axis and hence increased tilt of theplatform. It will be appreciated that the employment of a constantvelocity joint will overcome this problem. It was also found that thefriction about the two bearing axes of the joint differed and theplatform assumed a small though substantially constant tilt or list. Anantenna on the platform may be accurately set despite this list but itis preferred, of course, that the assembly be made symmetrical withsubstantially the: same total Coulomb friction torque about each of thetwo bearing axes.

What is claimed is:

l. A system for semi-stabilizing orientation of a directional receivingantenna of an antenna arrangement of a ship, wherein said systemcomprises: a structure having a mass of M kg and including said antennabalanced about a point fixed with respect to said structure; bearingmeans which define axes of rotation for said structure, which axesintersect at a pivotal center; and a radome covering said structure;said structure being, in use, gimbally mounted pendulously upon saidship by said bearing means with the center of gravity of said structurelocated below said pivotal center and spaced therefrom by a distancewhich, in meters, does not exceed 0.002 times the minimum value of thequotient I/M, where Ikg m is the moment of inertia of said structureabout any of said axes, and said bearing means has a maximum totalCoulomb friction torque about any of said axes which, in Newton-meters,is not greater than 0.00075 X l.

2. An antenna arrangement according to claim 1 wherein said bearingmeans comprises a universal joint.

3. An antenna arrangement according to claim 2 wherein said universaljoint is of the constant velocity type.

4. An antenna arrangement according to claim 1 including means foradjusting the location of said center of gravity relative to saidpivotal centre.

5. An antenna arrangement according to claim 1 wherein said totalCoulomb friction torque is of substantially the same magnitude about allof said axes.

1. A system for semi-stabilizing orientation of a directional receivingantenna of an antenna arrangement of a ship, wherein said systemcomprises: a structure having a mass of M kg and including said antennabalanced about a point fixed with respect to said structure; bearingmeans which define axes of rotation for said structure, which axesintersect at a pivotal center; and a radome covering said structure;said structure being, in use, gimbally mounted pendulously upon saidship by said bearing means with the center of gravity of saId structurelocated below said pivotal center and spaced therefrom by a distancewhich, in meters, does not exceed 0.002 times the minimum value of thequotient I/M, where Ikg m2 is the moment of inertia of said structureabout any of said axes, and said bearing means has a maximum totalCoulomb friction torque about any of said axes which, in Newton-meters,is not greater than 0.00075 X I.
 2. An antenna arrangement according toclaim 1 wherein said bearing means comprises a universal joint.
 3. Anantenna arrangement according to claim 2 wherein said universal joint isof the constant velocity type.
 4. An antenna arrangement according toclaim 1 including means for adjusting the location of said center ofgravity relative to said pivotal centre.
 5. An antenna arrangementaccording to claim 1 wherein said total Coulomb friction torque is ofsubstantially the same magnitude about all of said axes.